1. Field of the Invention
The present invention is directed to an infrared lamp with a closed-off enveloping tube which encloses an emission source joined with contacts for a power supply in the form of a carbon ribbon which extends in the direction of the long axis of the enveloping tube and determines an irradiation length of the infrared lamp. Furthermore, the present invention is directed to a procedure for heating a material to be processed using an infrared lamp which permits a heating rate of at least 250xc2x0 C./second.
2. Discussion of the Background
An infrared lamp is known from GB-A 2 233 150 in connection with which the emission source is constructed in the form of an elongated carbon ribbon which extends from one face to an opposite face of a quartz glass enveloping tube closed at both ends. The carbon ribbon includes a great number of graphite fibers arranged parallel to one another and in the form of a ribbon. For electrical contact, the carbon ribbon is provided with metal end caps on both sides. Usually, the ends of the carbon ribbons are clamped into the end caps. The caps are joined with a metal wire bent into a spiral, which engages on an electrical bushing projecting through closed faces of the enveloping tube. The irradiation length of the infrared lamp results directly from the length of the carbon ribbon.
The carbon ribbon allows a rapid temperature change of at least 250xc2x0 C./second, so that the background infrared carbon lamps are distinguished by a high rapidity of reaction. Nonetheless, the radiation output of a radiating body greatly depends upon its temperature in accordance with the Stefan-Boltzmann Law,xe2x80x94i.e. it recedes considerably with diminishing temperature. The background carbon lamp can indeed be used at high temperatures around 1450 K. In this case, however, it should be assured that the quartz glass enveloping tube does not come into contact with the hot carbon ribbon. In contrast, if the carbon lamp is operated at temperatures below the load limit of quartz glass (about 1270xc2x0 K), then the radiation output diminishes according to the Stefan-Boltzmann Law.
One object of the present invention is to provide a novel infrared lamp which can increase radiation output.
A further object of the present invention is to provide a novel procedure for the use of an infrared lamp for processing material layers which facilitate short treatment times with a simultaneously high degree of energy efficiency.
With respect to the novel infrared lamp, the present invention achieves the above and other objects by providing a novel infrared lamp in which a carbon ribbon has a length which is greater than an irradiation length by at least a factor of 1.5.
xe2x80x9cIrradiation lengthxe2x80x9d is understood to mean the longitudinal segment of the infrared lamp which contributes directly to heating. This longitudinal segment extends between the ends of the enveloping tube which are not heated. While with a background infrared lamp the length of the carbon ribbon corresponds to the irradiation length, the length of the carbon ribbon of the infrared lamp of the present invention is at least 1.5 times as long as the irradiation length. In this way, in the present invention an enlargement of the emitting surface over the irradiation length by the factor of 1.5 is attained, resulting in a corresponding increase in irradiation output in connection with the same surface temperature, according to the Stefan-Boltzmann Law. Consequently, with the novel infrared lamp of the present invention, high output densities are attainable even at low operating temperatures, e.g., at least 15 Watts per cm3 of the volume enclosed by the enveloping tube over its irradiation length.
The higher output density achieved in the present invention has very advantageous results in several respects. The infrared lamp of the present invention permits rapid heating of at least 250xc2x0 C./second and rapid cooling, and consequently behaves, with respect to its rate of temperature change, similarly to short wave infrared lamps. The maximum emission, however, of short wave infrared lamps usually lies in the wavelength range between 0.9 xcexcm and 1.8 xcexcm. In contrast, with the novel infrared lamp of the present invention, the maximum emission may lie in the wavelength range from about 2.3 xcexcm to 2.9 xcexcm due to the lower operating temperatures below about 1220 K. This wavelength range agrees with the wavelength range of about 1.8 xcexcm to 4 xcexcm, within which water-containing processing material has its maximum absorption. Due to the increased irradiation output of the novel infrared lamp of the present invention, a comparatively low energy rate suffices for operating the novel infrared lamp in this wavelength range. This leads to a corresponding low heating of the lamp surroundings. Consequently, it surprisingly appears that the efficiency with infrared treatment of a conventional processing material can be improved with the novel infrared lamp of the present invention, and that the energy requirement can at the same time be lower than with background short wave infrared lamps.
Enlarging the surface of the carbon ribbon in comparison with a simple elongated construction is achieved in the present invention through special geometrical shaping of the carbon ribbon, such as by folding, bending, rolling, or twisting the carbon ribbon. The length of the carbon ribbon corresponds at most to 66.67% of the length of the carbon ribbon in its elongated form after this shaping.
A carbon ribbon with a spiral construction has proven especially advantageous. As a consequence of the spiral shape, the surface of the emission source is significantly larger than the surface of a cylinder-shaped extended ribbon of equal length. With the spiral shape, the outward radiating surface is relevant for the power output which, apart from the gap between the windings, has approximately the shape of a cylindrical casing surface. In this case, it is important in the sense of the present invention that the surface radiating outward be larger than the irradiation length by at least a factor of 1.5. The larger surface once again leads to a higher irradiation output at a given surface temperature.
In equally preferred embodiments, the carbon ribbon can be folded like an accordion or bent into a wave-like shape. It is important that such special shapes result in a length of the carbon ribbon which is larger than the irradiation length by at least a factor of 1.5. The thickness of the carbon ribbon usually lies in the range between 0.1 mm and 0.5 mm, and its width in the range between 2 mm and 2.5 mm.
With respect to the procedure for heating the material to be processed using an infrared lamp, the objective indicated above is accomplished in that the novel infrared lamp of the present invention is operated such that its maximum emission lies at a wavelength ranging from 1.8 xcexcm to 2.9 xcexcm, and such that its power output reaches at least 15 Watts per cm3 of the volume enclosed by the enveloping tube over its irradiation length.
Heating a treatment material by the infrared lamp can, for example, result in drying, hardening, softening, or fusing. The indicated wavelength from 1.8 xcexcm to 2.9 xcexcm goes along with a surface temperature in the range from about 1250 K to about 1000 K. Owing to the comparatively large surface of the emission source, high output densities are attainable with the novel infrared lamp even at these relatively low operating temperatures. In accordance with the present invention, a power output of at least 15 Watts per cm3 of the volume of the enveloping tube enclosed over the irradiation length is set for heating the treatment material, whereby this power output basically includes a wavelength range from about 1.8 xcexcm to 4 xcexcm, within which a water-containing treatment material usually has its maximum absorption. For the operation of the novel infrared lamp, therefore, not only is comparatively low energy use achieved, but in particular its wavelength range accords well with an application-specific wavelength range of about 1.8 xcexcm to 4 xcexcm. In this way, the irradiation durations for the desired heating are short. With such a mode of operation of the novel infrared lamp, the degree of effectiveness for heating a treatment material is consequently better than with background short wave infrared lamps. In particular, the energy requirement for heating is lower and the treatment duration is shorter.
A procedure is especially preferred in connection with which the maximum emission wavelength ranges from 2.3 xcexcm to 2.7 xcexcm. With an operating mode of the novel infrared lamp of the present invention in this wavelength range, an especially high degree of energy efficiency with a short treatment duration is attained.